Method of forming a sapphire crystal using a melt fixture including thermal shields having a stepped configuration
Various single crystals are disclosed including sapphire. The single crystals have desirable geometric properties, including a width not less than about 15 cm and the thickness is not less than about 0.5 cm. The single crystal may also have other features, such as a maximum thickness variation, and as-formed crystals may have a generally symmetrical neck portion, particularly related to the transition from the neck to the main body of the crystal. Methods and for forming such crystals and an apparatus for carrying out the methods are disclosed as well.
1. A method of forming a sapphire single crystal comprising:
providing a melt fixture comprising:
a crucible;
a die having a length, wherein the die is open to and extends along a length of the crucible; and
a plurality of thermal shields overlying a crucible lid and adjacent to the die, wherein:
each of the thermal shields has a length, a width, and a thickness, wherein length>width>thickness;
the thermal shields include a lower thermal shield and an upper thermal shield;
the lower thermal shield is disposed between the crucible lid and the upper thermal shield; and
the upper thermal shield has a length that is shorter and extends along a smaller fraction of the length of the die than a length of the lower thermal shield, such that a combination of the lower and upper thermal shields defines a stepped configuration with respect to the length of the die;
forming a melt in the crucible; and
drawing an as-formed sapphire single crystal sheet from the die.
2. The method of claim 1 , wherein thermal shields include a first shield set positioned along a first lateral side of the die, and a second shield set positioned along an opposite, second lateral side the die.
3. The method of claim 2 , wherein each of the first and second shield sets is generally symmetrical about a vertical central axis corresponding to the midpoint of the die.
4. The method of claim 1 , wherein from a side view, the thermal shields have an overall shape that is generally trapezoidal.
5. The method of claim 1 , wherein thermal shields are substantially parallel to one another.
6. The method of claim 5 , wherein the melt fixture further comprises shield pins to hold the thermal shields in place.
7. The method of claim 1 , wherein the thermal shields provide a static temperature gradient along the die.
8. The method of claim 7 , wherein the thermal shields provide a temperature profile along the length of the die such that the temperature profile has a maximum temperature at about a midpoint of the die.
9. The method of claim 1 , wherein the as-grown single crystal sheet has a neck and a main body that defines first and second opposite lateral sides that are generally parallel to each other, a transition of the neck to the main body portion is defined by respective first and second transition points of the first and second opposite lateral sides, and the single crystal has a Δ T not greater than 4.0 cm, wherein Δ T is the distance by which the respective first and second transition points are spaced apart as projected along a length segment of the single crystal sheet.
10. The method of claim 1 , wherein the sapphire single crystal sheet has a length, width and thickness, wherein length>width>thickness, the width is not less than 28 cm, and the thickness is not less than 0.5 cm.
11. The method of claim 10 , wherein the width is not less than 28 cm.
12. The method of claim 10 , wherein the sapphire single crystal sheet has a variation in thickness is not greater than 0.2 cm.
13. The method of claim 1 , further comprising pulling the single crystal upward from the die and into an afterheater having a lower compartment and an upper compartment separated from each other by an isolation structure.
14. The method of claim 13 , further comprising cooling the single crystal in the upper compartment of the afterheater, the single crystal cooling at a rate not greater than 300° C./hr.
15. The method of claim 13 , wherein the single crystal is a generally planar sheet having a mass greater than 4 kg.
16. The method of claim 15 , wherein the isolation structure comprises isolation doors, and wherein the method further comprises closing the isolation doors after the crystal passes into the upper compartment.
17. The method of claim 1 , further comprising machining the as-formed sapphire single crystal sheet.
18. The method of claim 17 , wherein machining comprises grinding, lapping, or polishing.
19. The method of claim 17 , wherein machining comprises wire sawing or cleaving.
20. The method of claim 17 , wherein after machining, the sapphire single crystal sheet is transparent to radiation within the infrared and visible wavelength spectrums.